1. Field of Invention
This invention relates to the preparation of a Ziegler-type catalyst having a purple color. The catalyst of the present invention is particularly useful in the polymerization of .alpha.-olefins to produce polyolefin powders.
2. Description of Prior Art
It is known that TiCl.sub.4 can be reduced with alkyl aluminum halides. Thus the reduction of TiCl.sub.4 by Et.sub.2 AlCl or EtAlCl.sub.2 has been described by this author in U.S. Pat. No. 3,905,915. Reduction of mixtures of aluminum alkyl halides is also known and has been described in various patents, of which the following serve as examples: In U.S. Pat. No. 3,397,195 assigned to Hercules, 1 mole of TiCl.sub.4 is reduced with a mixture comprising 0.8 moles EtAlCl.sub.2 and 0.8 moles Et.sub.2 AlCl; in U.S. Pat. No. 3,388,076 assigned to Hercules, one mole of TiCl.sub.4 is reduced with a mixture comprising 1.2 moles of EtAlCl.sub.2 and 1.2 moles of Et.sub.2 AlCl; in U.S. Pat. No. 2,909,511 assigned to Hercules, one mole of TiCl.sub.4 is reduced with a mixture comprising 0.86 moles of EtAlCl.sub.2 and 0.6 moles of Et.sub.2 AlCl.
It is known in the art that the reduction product of TiCl.sub.4 and Et.sub.n AlCl.sub.3-n, where n can have values from 2-3, is the kinetically favored beta-form which has a brown color. This beta-TiCl.sub.3 can be converted to the thermodynamically favored purple form which can have either the alpha, gamma or delta structure. This crystal conversion, which is necessary to obtain a stereospecific polymerization catalyst, can be effected by heating the catalyst at temperatures near 150.degree. C as has been described in Br. Pat. No. 1,139,450, assigned to Shell, or by treating the brown TiCl.sub.3 form with excess TiCl.sub.4 as has been described in U.S. Pat. No. 3,769,233, assigned to Solvay.
It has recently been described in the art that TiCl.sub.3 compounds containing Et.sub.n AlCl.sub.3-n, where n can have values from 0.1 to 2, can be activated by sequential Lewis base-TiCl.sub.4 treatments. Thus Solvay (DT Pat. No. 2,213,086, South African application No. 72-01,245) published that TiCl.sub.3 catalysts formed by the reduction of TiCl.sub.4 with alkyl aluminum compounds at moderate temperatures can be substantially improved by subsequent treatments with certain Lewis bases and TiCl.sub.4. The subsequent treatment appears to effect substantial improvements in efficiency and stereospecificity as compared to catalyst not so treated.